How can we measure the differential current? Simply by an overcurrent relay. Our expectations are: 1- No operation for normal duty of the device. SIPROTEC 4 7UT6 Differential Protection Relay for Transformers – Connection of transformer differential protection with high impedance REF (I7) and neutral. A differential relay is defined as the relay that operates when the phase difference of two or more identical electrical quantities exceeds a predetermined amount. • The differential relay works on the principle of comparison between the phase angle and magnitude of two or more similar electrical quantities.


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It allows investigations into protection of transformers, autotransformers, generators and other electrical apparatus with two windings. The relay shows clearly the characteristics of three-phase differential protection relay protection.

For the protections of the feeders, the voltage balance differential relays are used.

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In this arrangement, the two similar current transformers are connected at either end of the system element under protection using pilot wires. The relays are connected in series with the pilot wires, one at each end. The relative polarity of the current transformers is such that there is no current through the relay under normal operating conditions and under fault conditions.

Differential protection relay CTs used in such protections should be such that they should induce voltages in the secondary linearly with respect to the current.


Since the magnitude of the fault current is very large, so that the voltage should be a linear function of such large currents, the CTs should be aired cored.

When the fault occurs in the protected zone, the currents in the two primaries will differ from one another, and so voltage induced in the secondaries of the CTs will differ and circulating current will flow through the operating coils of the relays.

The differential relay generates a tripping signal if the differential current, Id, is greater than a percentage of the restraining current, IRT: Elmareimi and Rekina Fouad Format: The connection should be such that, the induced voltage in secondary coil of one relay will oppose same of other.

The compensating device neutralizes the effect of pilot wires capacitance currents and effect of inherent lack of balance between the two current transformers.

Consequently, the EMF induced in the secondaries of the relay is also same but the coils are so connected, these EMFs are in opposite direction. As a result, no current will flow through the pilot loop and thereby no operating torque is produced either of the relays.

But if any fault occurs in the feeder within the zone in between current transformers, the current leaving differential protection relay feeder will be different from the current entering into the feeder.

Consequently, there will be no equality between the currents in both CT secondaries. These unequal secondary CT currents will produce unbalanced secondary induced voltage in both of the relays.


Therefore, current starts circulating in the differential protection relay loop and hence torque is produced in both of the relays. As the direction of secondary current is opposite into relays, therefore, the torque in one relay will tend to close the trip contacts and at the same time torque produced in other relay will tend to hold the movement of the trip contacts in normal un-operated position.


The voltage differential relay uses two similar current transformer places across the protective zone with the help of pilot wire. The relays are connected in series with the secondary of the current transformer.


The relays are connected in such differential protection relay way that no current flows through it in the normal operating condition. The voltage balance differential relay uses the air core CTs in which the voltages induces regarding current.

The current starts flowing through the operating coil.

Thus, the relay starts operating and gives the command to the circuit breaker to operates. Numerical[ edit ] The distinction between digital and numerical protection relay rests on points of fine technical detail, and is rarely found in areas other differential protection relay Protection [28]: Ch 7, pp Numerical relays are the product of the advances in technology from digital relays.

Generally, there are several different types of numerical protection relays. Each type, however, shares a similar architecture, thus enabling designers to build an entire system solution that is based differential protection relay a relatively small number of flexible components.

For example, a relay including function 51 would be a timed overcurrent protective relay. Overcurrent relay[ edit ] An overcurrent relay is a type of protective relay which operates when the load current exceeds a pickup value.